Internal combustion engines of a vehicle may operate in various combustion modes. One example mode is spark ignition (SI), where a spark performed by a sparking device is used to initiate combustion of an air and fuel mixture. Another example mode is homogeneous charge compression ignition (HCCI), where an air and fuel mixture achieves a temperature where autoignition occurs without requiring a spark from a sparking device. In some conditions, HCCI may have greater fuel efficiency and reduced NOx production compared to SI. However, in some conditions, such as with high or low engine loads, it may be difficult to achieve reliable HCCI combustion.
One approach in this field is US Pub. No. 2005/0173169, which uses a dual combustion mode engine configured in a hybrid vehicle propulsion system. Specifically, the engine is configured to operate in HCCI mode under some conditions and SI mode under some conditions, where the hybrid system may be used in conjunction with the engine to reduce transitions between combustion modes and provide the requested output to the vehicle drive wheels. Further, the engine output level is controlled based on the amount of stored energy.
However, the inventors herein have recognized a disadvantage with such an approach. Specifically, when trying to decrease engine mode transitions using the hybrid system and simply controlling engine output in response to the amount of stored energy, inefficient operation may result. For example, the available engine power output levels may vary depending on the combustion mode, and thus the engine may be operated in an inefficient manner in order to respond to the amount of stored energy while attempting to avoid mode transitions. In other words, the power output levels of the HCCI mode may be different than those of the spark ignition mode, and thus if the hybrid system is used to maintain a given combustion mode, the engine may be forced to operate inefficiently in order to provide the desired engine output as determined from the amount of stored energy.
Thus, it may be advantageous, at least under some conditions, to adjust combustion modes in a hybrid vehicle in response to the amount of stored energy to provide improved engine operation while still meeting the requirements of the hybrid system. While this may result in increased transitions under some conditions, improved overall hybrid system operation may be achieved. For example, under some conditions, the hybrid system may be used to reduce transitions (e.g., when sufficient energy storage is available), but under other conditions (e.g., lower levels of energy storage), the hybrid system may actually create a need to transition combustion modes to enable efficient operation or to produce the requested driver output.
In one particular embodiment, a hybrid vehicle propulsion system is provided. The system comprises: an engine having at least one combustion cylinder configured to operate in a first and a second combustion mode, wherein the first combustion mode is a spark ignition mode and the second combustion mode is a compression ignition mode, wherein the engine is configured to supply an engine output; a motor configured to selectively supply or absorb a secondary output; an energy storage device configured to selectively store at least a portion of the engine output and selectively supply energy to the vehicle; and a control system configured to control at least the engine, the motor, and the energy storage device, wherein the control system is configured to selectively operate said at least one combustion cylinder in one of the spark ignition mode and the compression ignition mode based on a condition of the energy storage device.